As a packaging method of an acoustic wave device, there has been known a method that face-down mounts a chip on a circuit board, and then covers the periphery of the chip by a sealing member. The integration and downsizing of the acoustic wave device have been desired. Japanese Patent Application Publication No. 2008-546207 (Patent Document 1) describes that two substrates each including an acoustic wave element formed on the surface thereof are bonded together through an interlayer so that the acoustic wave elements face each other across an air gap.
In the method that stacks the substrates each including a functional element such as an acoustic wave element formed thereon as described in Patent Document 1, the stress between the substrates becomes an issue. For example, the thermal stress between the substrates causes the strain of the substrate. The strain of the substrate deteriorates the characteristics of the functional element. In addition, when the stress between the substrates is intended to be reduced, the heat release performance from the acoustic wave elements or the like decreases.